1. Field of the Invention
The present invention relates generally to the field of optical communications systems. More specifically, the present invention discloses an optical wavelength router for wavelength division multiplex (WDM) optical communications.
2. Statement of the Problem
Wavelength division multiplexing is a commonly used technique that allows the transport of multiple optical signals, each at a slightly different wavelength, over an optical fiber. The ability to carry multiple signals on a single fiber allows that fiber to carry a tremendous amount of traffic, including data, voice, and even digital video signals. As an example, the use of wavelength division multiplexing permits a long distance telephone company to carry thousands or even millions of phone conversations on one fiber. By using wavelength division multiplexing, it is possible to effectively use the fiber at multiple wavelengths, as opposed to the costly process of installing additional fibers.
In wavelength division multiplexing techniques, multiple wavelengths can be carried within a specified bandwidth. It is advantageous to carry as many wavelengths as possible in that bandwidth. International Telecommunications Union (ITU) Draft Recommendation G.mcs, incorporated herein by reference, proposes a frequency grid which specifies various channel spacings including 100 GHz and 200 GHz. It would be advantageous to obtain 50 GHz spacing. Separating and combining wavelengths with these close spacings requires optical components which have high peak transmission at the specified wavelengths and which can provide good isolation between separated wavelengths.
One technique which has been developed to accomplish the demultiplexing of closely spaced wavelengths is to cascade a series of wavelength division demultiplexing devices, each device having different wavelength separating characteristics. A typical application involves cascading an interferometric device such as an arrayed waveguide device having a narrow spacing of transmission peaks (e.g., 50 GHz) with a second interferometric device which has a coarser spacing and correspondingly broader transmission peaks (e.g., 100 GHz spacing). The cascade of devices provides the required separation of wavelengths by subdividing the wavelengths once in the first device, typically into a set of odd and even channels, and then separating wavelengths in the subsets in following devices in the cascade.
Arrayed waveguide, fused biconical taper, fiber Bragg grating, diffraction grating, and other interferometric wavelength demultiplexing devices can be constructed to have the appropriate characteristics for the first or second stage devices in the cascade. However, traditional interferometric devices have the characteristic that as the spacing of the channels is decreased, the transmission peaks become narrower, and are less flat over the wavelength region in the immediate vicinity of each peak than a device with wider channel spacings. As a result, when using a traditional device in the first stage of a cascade, the transmission peaks may not have a high degree of flatness, and any drift or offset of a wavelength from its specified value may result in significant attenuation of that wavelength. In addition, the isolation between wavelengths is frequently unsuitable with conventional interferometric devices and can result in unacceptable cross-talk between channels.
With increasing numbers of wavelengths and the close wavelength spacing which is utilized in dense wavelength division multiplexing systems, attenuation and cross-talk must also be closely controlled to meet the system requirements and maintain reliable operations. As an example, 40 or 80 wavelengths can be generated using controllable-wavelength lasers, with transmission signals modulated onto each laser. It is desirable to be able to demultiplex these channels. Although the lasers can be controlled and the wavelengths stabilized to prevent one channel from drifting into another, there is always some wavelength drift which will occur.
For the foregoing reasons, there is a need for a wavelength division demultiplexing device which tolerates wavelength drift, maintains a high degree of isolation between channels, and is able to separate large numbers of wavelengths.
3. Prior Art
FIG. 1 illustrates a prior art interferometer that shares some of the basic principles employed in the present invention. An input laser beam is split into two beams by a beamsplitter 10. One beam propagates toward a mirror 14 and is reflected back by this mirror. The other beam propagates toward a resonator 12 and is also reflected back. The resonator 12 is a Fabry-Perot cavity (or etalon) with a partially-reflective front mirror and a totally-reflective back mirror. The resonator 12 reflects substantially all of the incident optical power back regardless of wavelength, but the group delay of the reflected light is strongly dependent on wavelength. The two reflected beams from the mirror 14 and from the resonator 12 interfere at the beamsplitter 10 and the resulting output is split into two beams, one at output A, and the other in a different direction at output B. The two output beams contain complementary subsets of the input optical spectrum, as shown for example in FIG. 2. Such a wavelength router concept has been proposed by B. B. Dingle and M. lzutsu, xe2x80x9cMultifunction Optical Filter With A Michelson-Gires-Tournois Interferometer For Wavelength-Division-Multiplexed Network System Applications,xe2x80x9d Optics Letters, vol. 23, p. 1099 (1998) and the references therein.
The two output ports A and B divide the spectral space evenly with alternating optical channels being directed to each output port (i.e., optical channels 1, 3, 5, 7, etc. are directed to output port A, while channels 2, 4, 6, etc. are directed to output port B). This function has sometimes been called an optical interleaver.
4. Solution to the Problem
The present invention address the problems associated with the prior art using a polarization-based interferometer to implement an optical interleaver capable of separating closely spaced optical channels with minimal cross-talk.
This invention provides a method and apparatus for optical wavelength routing in which an input beam is converted to a pair of orthogonally-polarized beams. A split-mirror resonator has a front mirror with two regions having different reflectivities, and a reflective back mirror spaced a predetermined distance behind the front mirror. Each of the orthogonally-polarized beams is incident on a corresponding region of the front mirror of the resonator. A portion of each beam is reflected by the front mirror, while the remainder of the beam enters the resonator cavity where it is reflected by the back mirror back through the front mirror. The group delay of each reflected beam is strongly dependent on wavelength. The two reflected beams from the resonator are combined and interfere in a birefringent element (e.g., a beam displacer or waveplates) to produce a beam having mixed polarization as a function of wavelength. The polarized components of this beam are separated by a polarization-dependent routing element (e.g., a polarized beamsplitter) to produce two output beams containing complementary subsets of the input optical spectrum (e.g., even optical channels are routed to output port A and odd optical channels are routed to output port B).
Alternatively, the split-mirror resonator can be replaced by a mirror and Fabry-Perot resonator. In this embodiment, one of the orthogonally-polarized beams is reflected by the mirror, while the other is reflected by the Fabry-Perot resonator.
These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawings.